Various polymers are used in the manufacture of electronic devices, to include photoresists and organic-based dielectrics. Photoresists, for example, are used throughout semiconductor device fabrication in photolithographic operations. The resist is exposed to actinic radiation through a photomask. Where a positive-acting resist is used, exposure causes a chemical reaction within the material resulting in a solubility increase in aqueous alkali, allowing it to be dissolved and rinsed away with developer. In the case of a negative-acting material, crosslinking of the polymer occurs in the exposed regions while leaving unexposed regions unchanged. The unexposed regions are subject to dissolution and rinsing by a suitable developer chemistry. Following development, a resist mask is left behind. The design and geometry of the resist mask is dependent upon the positive or negative tone of the resist; positive tone resist will match the design of the photomask, while a negative tone resist will provide a pattern that is opposite the photomask design. The use of photoresists requires several cleaning steps with a final clean of the mask before the next circuit design process step is implemented.
Organic-based dielectrics represent engineering polymers used to offer insulative properties to the microelectronic circuit. Examples of these chemistries include polyimide (PI) and poly-(p-phenylene-2,6-benzobisoxazole) (PBO) as manufactured by Hitachi-DuPont Microsystems. Another popular organic insulator for electronic applications is bisbenzocyclobutene (BCB), manufactured by the USA-based, Dow Chemical Company. These polymers are applied to the substrate in a similar fashion as photoresists using conventional spin, spray, or they may be slit-coated as is common practice in manufacturing FPDs. For these application reasons, organic-based dielectrics may often be referred to as spin-on dielectrics. Once the polymer is applied, they may undergo a patterning process, but ultimately all of these systems lead to a final-stage cure, which permanently fixes the material in place by undergoing chemical and physical property changes. The final material exhibits both electrical and physical properties desirable for performance of the electric circuit. Once these organic-based dielectrics are fully cured, they are considered to be permanent, whereby, the need for rework would either require the use of aggressive materials such as strong acids or bases that likely would attack the substrate or adjacent metals or more practically, the rework condition would be considered as not commercially available.
Positive photoresists are commonly based upon resins of the novolac or polyhydroxystyrene (Phost) varieties chosen for high-resolution device processing in front-end semiconductor and flat panel display manufacturing. Positive-tone systems represent the largest volume portion of photoresists produced globally and there are many suppliers. Example suppliers of these systems for both semiconductor and FPD include the USA-based AZ Electronic Materials, the USA-based Rohm and Haas Corporation, and the Japanese company, Tokyo Ohka Kogyo Co Ltd. In positive photoresist applications, a substrate is etched by plasma processes, which use gases of inert and chemical varieties to produce both ionized and reactive species that travel through the mask and etch down into the substrate. During etching, ionized and reactive species combine with atoms of the substrate, form a by-product, and that by-product is vented away via the reduced pressure of the plasma system. These same gaseous species also impact the photoresist mask, baking it into place and also ejecting carbon-containing by-products into the plasma. Photoresist by-products mix with other species in the plasma and are continually directed down towards the substrate. These materials condense to form a residue along the sidewalls of the etched features, producing a desirable condition, otherwise referred to as anisotropic etching whereby species are highly controlled and directed into the substrate with little or no lateral loss. Upon completion, it is desired to remove this etch residue along with the resist mask, as they can have deleterious effects on subsequent processes and lead to reduced device performance or device failure. Such residues and their associated resist masks, however, can be difficult to remove, normally involving the use of formulated stripper chemistries.
Negative photoresists are commonly chosen for more rigorous process conditions whereby more aggressive chemical or thermal exposure processes may be used. These negative photoresists include isoprene (rubber), acrylic, and epoxy-based resins. Cyclized isoprene (rubber) photoresists are chosen for their high chemical resistance. Examples of these photoresists may be obtained from Fujifilm Electronic Materials, Ltd. under the trade name SC-Resist or HNR-Resist. Negative-tone isoprene resin resists are common in aluminum processing where a brief chemical etch is used to remove metal surrounding the masked feature. Negative-tone acrylic photoresists are commonly chosen for wafer-level-packaging bump formation. Suppliers include the USA-based Printed Circuits Division of E. I. duPont de Nemours and Company under the trade name Riston, and the Japan's JSR Corporation for dry-film and spin-on (wet) negative acrylics, respectively. Dry-film and spin-on acrylics offer an ability to deposit thick layers from 25 to 120 microns (um), used to pattern the corresponding solder bumps. Once the pattern is formed, metal deposition occurs by electroplating or screen-printing, a process that exposes the resist to heated acid or baking in excess of 250° C., respectively. Another popular negative resist, an epoxy system under the trade name of SU-8™, originally developed by International Business Machines (IBM) and now sold by the USA company, MicroChem Corporation, and Gersteltec Engineering Solutions, a Swiss-based company. The SU-8™ is commonly chosen for thick patterns that may exceed 300 microns (um), with a high-aspect ratio (i.e. height vs width), and with the pattern definition to exhibit extremely straight sidewalls. Because of the extremely unique characteristics of the SU-8™ epoxy resin, photoresists of this variety are chosen to manufacture large devices, and most commonly include microeletromechanical systems (MEMS). The varieties of negative-tone photoresists are significantly different from positive, their cleaning (removal) practice is even more rigorous. In fact, it is commonly understood that SU-8™ photoresist is considered to be a permanent system, removed only with more complex, time, and costly practices.
As with any process involving photolithography, it is desirable to completely remove the photoresist from the substrate in order to proceed successfully to the next process. Incomplete stripping of the photoresist can result in irregularities during the next etching or deposition step, which may cause quality and yield problems. For example, during solder bumping, resist contamination can prevent metal solder from wetting to a metal pad during the board assembly reflow processes, resulting in yield loss in a finished assembly. The same photoresist contamination is manifested as organic contamination in front end of line device patterning and results in the exact same non-wetting problems in an etch or deposition process. Such irregularities, no matter how small, continue to magnify the problem throughout manufacturing until during final device assembly and testing, the condition leads to poor mechanical and electrical contacts, which produce high resistance and heat, or worse, catastrophic electrical shorting.
Throughout each of these chemical processes, one can appreciate maximum selectivity in cleanliness and high throughput must be met without failure. Any problems associated with a lack of performance, presence of residue, or worse, a rise in process complexity, all will result in reduced yield and increased cost.
It is generally understood that the chemistry of positive tone resists are typically hydrophilic (polar) and amorphous (i.e. non thermoset and cross-linked), and it is for these reasons that these systems are believed to be easier to clean (remove) using conventional solvents and/or chemical strippers. The resins for positive-tone chemistries are based upon either novolac (cresol, phenol-formaldehyde) or polyhydroxystyrene (PHost), with occasional options of styrenated copolymer and/or acrylic/PMMA (polymethylmethacrylate). These chemistries offer good adhesion and fixing to a wide variety of surfaces while the hydroxyl groups present in the various forms of novolac (i.e. cresol, bis-phenol, etc.) provide intermolecular hydrogen bonding which aids in aqueous solubility. This condition combines during the photoconversion of the initiator diazonaphthoquinone (DNQ) in novolac systems, while in PHost systems, the acid catalyzed de-protection of the ester forms the more soluble alcohol. When used during normal operating conditions up to and including 100 degrees C., these systems remain soluble in polar solvents while their UV-exposure will produce counterparts that are soluble in aqueous-base.
As indicated here, the positive-tone resists are used as primary imaging masks for plasma-based etching. During this process, species in the plasma produce etch residue while exposing the mask to temperatures exceeding 150 degrees C. It is well known that etch residue (e.g. side wall polymer) is comprised of by-products of the plasma with organic constituents of photoresist. The chemistry of the residue may comprise constituents of the substrate, metal topography, and plasma gases, to include silicon, gallium, arsenic, boron, phosphate, titantium, tantalum, tungsten, copper, nickel, aluminum, chromium, fluorine, chlorine, as well as carbon containing compounds. In novolac systems which contain hydroxyl constituents, these elevated temperature exposure conditions will facilitate further reactions to form insoluble species. Such reactivity of hydroxyl groups with halides and active metals, especially in the heated and acidic conditions of a plasma, to produce alkyl halides, esters, and, in some cases, high molecular weight polymers is known (Morrison, R. T. and Boyd, R. N., Organic Chemistry, 3rd Ed., Allyn & Bacon, Inc., Boston Mass., Ch. 16 (1973)). Conventional cleaning of etch residue and overexposed photoresist masks resulting from the effects of hot plasma etching require the use of chemical strippers processed at elevated temperatures for extended periods of time dependent upon the process and tool.
Typical measurement used to predict stripping challenges of bulk resins includes thermal analysis determination of glass transition (Tg). Relatively unchanged Tg values are observed in positive-tone photoresists and similar amorphous systems (Fedynyshyn, T. et al., Proc. SPIE 6519, 65197-1 (2007)). Detectable increases of Tg in photoresists have been observed to be a function of the evaporative loss in solvent, which in turn, will depend upon the thickness of the photoresist coating. Most notable are observed increases in Tg with radiation and thermal exposure with polymer crosslinking (J. D. D'Amour et al., Proc. SPIE 5039, 966 (2003)). Such crosslinking of high temperature exposed novolac resins and negative-tone systems is consistent with the presence of higher molecular weight species as detectable by increased values of Tg.
Cleaning (removal) of photoresist etch residue and the mask use complex chemical strippers composed of organic solvents, amines, water, reducing agents, chelating agents, corrosion inhibitors, and surfactants. The reducing agent, hydroxylamine, has been cited extensively in the literature as a basic material which facilitates dissolution of photoresist and its residue while offering protection of underlying aluminum metal features. Common practice in using stripper chemistries involves delivery of large volumes of stripper to the substrate to be cleaned at a specific temperature for a given period of time.
As the industry continues to replace aluminum with copper to capture improved performance in their devices, the stripper chemistries must also be adjusted. Hydroxylamine may be acceptable for cleaning of aluminum devices; however, it is too aggressive for copper. Device architecture using copper and low-K (dielectric constant, K), e.g. Cu/Low-K, require fluorinated-based chemistries to remove silicon-laden etch residue. Amines and ammonia compounds are known to be complexing agents for Cu and are observed to etch (attack) copper metal. Additionally, fluorinated and glycol-based stripper chemistries are considered toxic and exhibit elevated viscosities.
Negative photoresists used in forming wafer bumping metallization masks generally include acrylic, styrenic, maleic anhydride or related monomers and copolymers. Such materials are used to produce photosensitive thick films. These photoresists are commonly referred to as “acrylic” polymer systems due to the pendant groups on the main polymer chains, which include vinyl groups common to acrylics. In general, the dry-film form of acrylic photoresists is chosen where exposure to rigorous process conditions is required. As a result of this exposure, the cleaning of dry-film masks and residues presents a stripper challenge. When a dry-film system is removed, the material is typically not dissolved. Rather, many chemical strippers interact with the material to cause lifting or peeling from the substrate, resulting in the generation of suspended insoluble flakes and particles. Such insoluble materials can lead to filter fouling and performance degradation in the processing tool. This can create a significant loss in productivity as a result of process tool downtime for maintenance. In addition, the failure to filter off or rinse away particles may result in the formation of residue on the final product and contribute to yield loss.
Resist stripping compositions that include aromatic quaternary ammonium hydroxide such as benzyltrimethylammonium hydroxide (BTMAH), a solvent such as an alkylsulfoxide, a glycol and a corrosion inhibitor and non-ionic surfactant do not completely remove many dry-film resists from a wafer surface. Similarly, compositions which use pyrrolidone-based solvents such as N-methylpyrrolidone (NMP) exhibit the same drawback in that they cannot achieve complete removal of many dry-film resists. In general, compositions which include a quaternary ammonium hydroxide as tetramethylammonium hydroxide (TMAH) in NMP do not completely dissolve many dry-film resist. As discussed above, incomplete dissolution produces particles that can become a source of contamination resulting in yield loss.
Similar experience is noted for negative-tone photoresist of the rubber-based resin variety. Stripper chemistries used to clean residue and masks resulting from rubber photoresists include a hydrocarbon solvent and an acid, commonly a sulfonic acid. High acidity is required for performance and emulsification of hydrolyzed rubber components. Representative inhibitors include mercaptobenzotriazole (MBT) and related triazoles to prohibit attack upon adjacent metallic features. A common inhibitor for these chemistries includes catachol, a toxic and carcinogenic material. Further, rinse steps for hydrocarbon strippers of this variety must use isopropanol (IPA) or related neutral and compatible solvents. This rinse practice, albeit a cost increase, will reduce the effects of metal attack to adjacent metals due to a pH-drop during water mixing with constituents of the stripper. Due to compatibility issues, wastes from the use of hydrocarbon-based strippers must be segregated from normal organic streams in a microelectronic fab.
While it is important to give attention to the challenges of polymer and residue removal from the standpoint of the stripper chemistry, equal diligence is necessary towards the design of the process and proper performance of the tool. It is generally understood that the primary purpose of the cleaning tool is to provide control in the process. Variability between part batches is reduced by the operation of the tool. Barring any mixing or chemical adjustments made by the unit, the variables available to the tool for control include temperature, agitation, and time. With an ever-present intensive pressure to increase throughput in a manufacturing line, a constant emphasis is to decrease the process time. Again, without a change in chemistry, this leaves as the only option to increase temperature and agitation, with the expectation that polymer dissolution rates will increase resulting in shorter process time. However, other reactions which are contradictory to the objectives of the process, such as corrosion rate, will also increase with increase in temperature and agitation. Additionally, and most important, there is continued loading of the stripper chemistry with the organic substance, causing a reduction in bath life and accelerates the observation of residue or other phenomena that indicate a drop in performance.
On the temperature continuum, bath life may be facilitated by increasing temperature or agitation. Where agitation must be controlled to protect substrate features, bath life conditions may be increased through increased polymer dissolution with increasing temperature. There is a fundamental safety limit as communicated by industry guidelines (SEMI S3-91, Safety Guidelines for Heated Chemical Baths). In accordance with SEMI, liquid over temperature shall be controlled at not more than 10 degrees C. above the normal operating temperature of the liquid, where the typical operating temperature does not exceed the flashpoint of the liquid. Many companies will set policy which is more restrictive such as operating at 10 degrees C. below the flashpoint and setting the over temperature to be the flashpoint. These criteria and others may best be observed in the processing of flat panel displays (FPDs).
Resist stripping at a FPD manufacturing plant occurs on large substrates traveling on a conveyor from one chamber to another. The resist is stripped from the panel by a stripper delivered by a sprayer that floods the entire glass surface, traveling to a rinse stage where distilled, deionized, or demineralized water or an alternative solvent is sprayed onto the surface, and the process is completed with a drying step that includes a hot air knife. Stripping is supported by at least two product tanks which are separate and distinct and arranged in-line with the flow direction of the parts. Substrates entering the tool will be first “washed” by the chemistry in the first tank. The stripper is sprayed onto the substrate surface, and upon reacting with the resist and flowing off of the substrate, it is collected and returned to the tank where it is subsequently heated and filtered such that any suspended and undissolved materials are removed from the bulk chemistry. The filtered and heated stripper is then cycled back to the spray chamber where it is delivered to the substrate in a continuous manner that optimizes the resist stripping process.
As the part travels on the conveyor from the first chamber supported by tank #1 to the next chamber supported by tank #2, there is a fundamental purity change in the stripper. Although the conditions of operation for tank #2 may be the same as that for tank #1, the amount of resist present is lower than that for tank #1. Typical processing times are defined for chamber #1 to offer a dwell time of the chemistry in contact with the resist which optimizes resist stripping and maximum removal. Over time, tank #1 will reach a maximum loading capacity for dissolved resist and a decision to replace the contents will be necessary. When this occurs, the contents of tank #1 will be sent to waste and replaced by the contents of tank #2. The contents of tank #2 will be replaced with fresh stripper (i.e. pure stripper). In this manner, the system is said to operate in a counter-current fashion. Namely, the process flow of parts is “counter” or opposite to the flow direction of the chemistry. By using this practice, tanks #1 & #2 become the dirty and clean tanks, respectively. In other words, the unwanted resist is concentrated in the front of the line while the cleanest chemistries remain near the end whereby after this point, the product substrate is rinsed and dried.
The configuration given here for the FPD example is consistent with most, if not all, in-line bench style tools and with many batch style-processing tools. In a bench tool, parts move from one station to another while the tanks are at fixed locations. In a batch style tool, the parts may rotate but remain at a fixed location, while the chemistry is being delivered by spraying. There will be two tanks, the tool will pump from one or the other and carry-out counter-current cleaning designs by the use of “dirty” and “clean” tanks.
There is an equal, yet unsatisfied, need to achieve selectivity during processing with these formulated strippers. Namely, as the use of more aggressive chemistries is put into practice to achieve a desired cleaning performance in ever reducing time, this practice must be met without damage to sensitive metals and the underlying substrate. This is especially challenging as many of the acids or alkalis of choice will rapidly “spike” the pH of the system, once they are mixed with water during the rinse step, causing galvanic corrosion to substrate metals. During the rinse stage on a FPD line, water is sprayed on the heated glass surface that contains residual stripper. No surfactants are used in a FPD line, in fear that a foam condition will occur and ultimately cause catastrophic failing of filters, pumping of dry air bubbles, and worse, contaminating the fab by overflowing stripper which may trigger electrical shorting and lead to a fire. Since no surfactants are used, there is irregular diffusion due to rising surface tension from the organic stripper to the aqueous condition. Irregular mixing and spreading cause momentary dead spots on the panel, which contribute to accelerated corrosion. The corrosive byproduct and foaming condition may be avoided through rinsing with neutral solvents such as isopropanol (IPA). Although this practice is accepted by several FPD manufacturers, it is both expensive and a flammability hazard.
There is a need, accordingly, for improved stripping compositions which will remove the processed resist in a rapid manner while maintaining safety towards the underlying metallurgy during rinsing with distilled, deionized, or demineralized water, and preventing corroding, gouging, dissolving, dulling, or otherwise marring the surfaces throughout the entire process. Further, growing initiatives exist within the industry to move towards being “green.” A green process and the associated chemistries are those which will reduce or eliminate the use and generation of hazardous substances. According to the American Chemical Society's Green Chemistry Institute, there are twelve (12) principles which help to define a green chemistry.
This review of polymeric substances in microelectronic fabrication presents serious and compelling challenges in the industry. Where organic dielectrics are used, there is a continuing need for processes and compositions which may be used to effectively re-work a cured polymer by dissolving and cleaning the unwanted material from the underlying substrate. In cases of positive photoresists, there is a similar and continuing need for processes and compositions to effectively remove polymer from a substrate without deleterious effects to adjacent metal features. Finally, in the case of negative-tone photoresists, the same need exists for processes and compositions to effectively remove polymer from a substrate without deleterious effects to adjacent metal features. Although all of these materials are organic in origin, their chemistry is different and presents unique challenges that must be overcome in order to effect the desired cleaning result.
While there is a desire to address the removal needs of organic substances with unique compositions, there also, is a challenge to design a process that is supported by a tool which will enable rapid processing of parts, rinsing with water, without deleterious effects to the substrate. There is a continuing emphasis for the microelectronics industry to be green through improving the safety of operations, reducing the use of chemistry, and reducing the generation of hazardous waste. Taking these challenges together, there is a pressing need to provide a consistent and universal process, which uses compositions of matter that vary depending upon the performance needs of the unique polymer or residue to be removed, which provides high performance, high throughput, a green process, all at a reduced cost of ownership.